The art on the optical methods of processing radar (RF) signals appears in these patents: U.S. Pat. No. 5,296,860 and RE. 36,944 of “Optical Fiber Based Bistatic Radar”; U.S. Pat. No. 5,294,930 and RE. 35,553 of “Optical RF Stereo”; and U.S. Pat. No. 5,955,983 “Optical Fiber Based Radars”. It is known to those of ordinary skill in the art that best methods in radar signal processing are to use a radar (RF) signal as reference to process other radar signals. These patents lead to the realization of the best methods. However, the art of these patents relies on coherent RF receivers, which are conventional radar receivers.
The art on the signal processing of a single pulsed or transient signal appears in these patents: U.S. Pat. No. 5,589,929 and RE. 37,561 of “RF Signal Train Generator and Interferoceivers”; U.S. Pat. No. 6,001,506 of “Different Models for RF Signal Train Generators and Interferoceivers”. A single pulsed or transient signal is short, and its contained information could not be completely captured prior to the above inventions. With the advent of RF signal train generator, which is able to store the short signal and regenerates its replicas for the purpose of repeated analyses, a new door opens to capture the complete information. The apparatus in these patents have been referred to as interferoceivers. However, the art of these interferoceivers relies on conventional RF receivers, which are well known to those of ordinary skill in the art. Interferoceivers can be incorporated with the art of the above optical methods of processing radar (RF) signals to improve their performance.
A simultaneous signal is a transient, non-cooperative, and complex radio frequency (RF) signal from various sources. It comprises multiple components over a wide range of frequencies. Conventional RF receivers in interferoceivers of prior art cannot capture complete information contained in a simultaneous signal. Hence, there is a need in the art for new interferoceivers to have the capabilities of capturing complete information contained in a simultaneous signal, and of deciphering its complexity.
Channelized receivers are well known in the prior art to analyze various simultaneous signals and to identify their complexities. These receivers are important and indispensable tools in intelligence gathering of electronic warfare to defeat hostile military operations. Such receivers are able to identify individual components in simultaneous signals. The frequency bandwidth of interest for simultaneous signals could be over 20 GHz. A prior art channelized receiver includes numerous local oscillators, and large number of narrow-band and contiguous filters.
It is difficult to fabricate the narrow-band filters in high frequencies. Simultaneous signals are down converted to a number of intermediate frequency (IF) bands with the help of local oscillators. Signal components are then classified according to IF bands. Components in each IF band are sorted out by narrow band filters. Leading and trailing edges of these components contain high frequency elements. Down conversion will filter out their high frequency elements and modify their characteristics. Although channelized receivers are able to determine frequencies, the characteristic properties of components in simultaneous signals are lost. This is a problem of the channelized receivers.
Due to the large number of filters and local oscillators, the prior art channelized receivers are expensive to fabricate, bulky in size, and difficult to maintain. The advancement of microchip based receivers and surface acoustical wave filters has eased some problems associated with fabrication and size. But it is still not able to overcome the problems of the large number of narrow-band filters, and the difficulty in configuring them. This is another problem for the channelized receivers.
The narrow-band filters operate in parallel. The incoming simultaneous signals have to be distributed into these filters. The distribution reduces the signal strength received by each individual filter. A large number of amplifiers must be installed to amplify the signal strength before distribution. This is not only expensive but also increases the noise and alters the characteristic properties of signal components. Channelized receivers could not overcome such degradation problems. This is a further problem for the channelized receivers.
In the prior art, the Bragg cell receiver can perform as a channelized receiver without hundreds of filters. The attractive feature of such a receiver is its potentially small size and low cost. Although its feasibility has been demonstrated, research and development are still needed to realize its full capability. The problems on the channelized receivers still remain the same.
Radar warning receivers are other wide band receivers in the prior art and closely related to channelized receivers. The objective of these receivers is on threat response and only searches radar signals in limited bands. Channelized receivers are for the reconnaissance and search signals in all RF bands. The division between these two types of receivers is due to the problems that channelized receivers are bulky, high cost, and not responsive. Otherwise, channelized receivers could be good electronic warfare receivers as well.
Spectrum analyzers are also well known in the prior art. The objective of these analyzers is very similar to that of channelized receivers in investigating simultaneous RF signals over a wide range of frequencies and to identify their characteristics. A typical spectrum analyzer includes a single super heterodyne receiver with a tunable oscillator to process the components in simultaneous signals. During its operations, the oscillator tunes through its range bands.
The difference between a spectrum analyzer and a channelized receiver is in their mode of operation. A spectrum analyzer operates in time sequence according to oscillator tuning steps. The simultaneous signals have to be cooperative and constantly available during the course of tuning steps. Such a requirement might be too demanding. Furthermore, the operation is time consuming. A channelized receiver operates instantaneously based on parallel processing of a large number of local oscillators, narrow band filters, and amplifiers. The difference in the mode of operation makes a spectrum analyzer much lower in cost than a channelized receiver.
Spectrum analyzers are good instruments in analyzing cooperative and continuously available signals, and can be found in most laboratories. Channelized receivers are specially designed apparatus and custom tailored to satisfy the needs of their users. Due to the sequential operation, the spectrum analyzer is not effective when signals are pulsed, simultaneous, and non-cooperative. The problem is that a spectrum analyzer could not function as a channelized receiver.
Sampling oscilloscope is another instrument known in the prior art. It uses an extremely fast sampler to sample transient signals, and can be found in most laboratories. Sampling oscilloscope is a good instrument in analyzing transient signals, which are short in pulse lengths, cooperative and easily reproducible. However, it is not effective when used as a channelized receiver, as the simultaneous signals are non-cooperative and irreproducible. In light of the above, the need in the art to have a low cost channelized receiver is not diminished with the existence of spectrum analyzers and sampling oscilloscopes.
RF receivers in interferoceivers of prior art are conventional radar receivers and RF digitizers. The former are super heterodyne receivers, which were originally invented for radio. A first requirement in super heterodyne receivers is the RF to IF down conversion with the help of local oscillators. Radar and radio signals are different. The former is pulsed and the latter continuous. Down conversion will not alter information contained in radio signals, but will drastically alter radar signals.
Leading and trailing edges of a radar signal contain rich higher frequency components. IF bandwidth is narrow and the local oscillator is not perfectly stable. After down conversion, high frequency components will be wiped out and the local oscillator injects more noises. Furthermore, down conversion introduces an arbitrary phase into the radar signal and can affect its coherence. These problems do not exist in the down conversion of radio signals.
Radar signals contain many intrinsic features, which may be unintended modulations of transmitters or intended modulations which are directly embedded. Hence real radar signals are not simple. Conventional radar receivers rely on matched filters at IF band in mimicking these features to achieve optimum detection of radar signals from targets. These filters might be able to mimic intended modulations, but they are incapable of mimicking unintended modulations and edge characteristics. Hence conventional radar receivers cannot lead to optimum detection of radar signals with the help of matched filters.
The task for detection of non-cooperative or simultaneous signals would be more difficult, as conventional RF receivers do not have a priori knowledge on their intrinsic features. Hence conventional RF receivers cannot lead to optimum detection of simultaneous signals.
RF digitizers have slow sampling rates and narrow sampling gates to digitize signals at RF level. From the digitizing principle of sample and hold, a RF digitizer has basic elements of a switch and a capacitor. During its operation and under the synchronization with the RF signal of interest, a gate pulse controls on and off of the switch and sets the location of the signal for the capacitor to be charged. The charge is then read as digitizing bits of the signal being sampled at the location. The sampling gate of a RF digitizer is determined by the width of the gate pulse, which in turn defines its frequency response. A short sampling gate leads to the low charge and low digitizing bits. This is a problem of RF digitizers.
Sampling oscilloscopes are based on RF digitizers. The above problem is also the problem for sampling oscilloscopes. In light of the above, the need in the art to improve sampling oscilloscopes is enhanced.
The deficiencies in conventional radar receivers and RF digitizers cause interferoceivers of prior art to be ineffective in most optimum, broadband, robust, and high precision detection of pulsed, transient, non-cooperative, complex, or simultaneous signals. These deficiencies are problems for interferoceivers of prior art.
In light of the above, we summarize that there is a need in the art for low cost, broadband, optimum, robust, high precision methods and apparatus of processing pulsed, transient, non-cooperative, non-reproducible, complex, or simultaneous signals to overcome above identified problems in interferoceivers of prior art, channelized receivers, electronic warfare receivers, spectrum analyzers, and sampling oscilloscopes.
Teaching of the Invention
The present invention is from the provisional patent application of “Robust and Broadband signal processing using Replica Generation Apparatus”, Application No. 60/614,046. Its teaching is from these three articles: Ming-Chiang Li, “A High Precision Doppler Radar Based on Optical Fiber Delay Loops” (IEEE Transactions on Antennas and Propagation, 2004, 52, pp. 3319-3328); Ming-Chiang Li, “New Channelized Receivers” (Transactions of the AOC, 2004, 1, pp. 76-97); and Ming-Chiang Li, “Radar Receivers Based on Correlation Measurements” (unpublished). Hence these articles are incorporated by reference herein.